OP can write this. I found sample of someone’s husband send similar letter regarding on GC interview ( no news after interview for over 120 days). Requested to be adjudicate or else will file mandamus.
attention field office director, sent to field office with certified mail. On the envelope write “ DO NOT OPEN IN MAILROOM”. One day after the letter received, I-485 approved
i think for N-400 issue OP had it can be applied, no?